The present invention relates in general to laser processing of workpieces such as semiconductor devices and more particularly concerns processing of DRAMS, memories, and programmable devices by cutting fuses or links.
Laser systems have been used for many years in the fabrication of DRAMS and programmable devices. In DRAM production, for example, redundant memory is programmed by using a focused laser beam to cut fuses or links in the memory in order to replace defective memory cells. The programming is accomplished by disconnecting the fuses or links using a laser pulse generated by a diode pumped Q-switched YAG (or YLF) laser.
Recent semiconductor devices have link geometries typically about 1 xcexcm wide by 5 xcexcm long. These links may be located in groups of horizontally aligned links and vertically aligned links. A laser having 3-5 xcexcm laser spot size may be used to disconnect such a link using a single laser pulse. By appropriately selecting the laser energy, the spot size, the laser pulse width, and the wavelength of the laser beam, it is possible to optimize laser parameters in order to achieve the cleanest and most reliable link disconnect.
The quality of a link disconnect may be evaluated by visually inspecting the blasted link. One measure of practicality in fuse or link disconnect is the energy cutting range or xe2x80x9cenergy window,xe2x80x9d which is the range of energies per pulse over which clean and reliable link cutting can be achieved. The laser energy that is used to process a semiconductor device can be set at the center of the predicted energy window, which may differ somewhat from the actual energy window due to process variations such as the thickness of the link material; the thickness of oxide material located on top of the link, laser instability, errors in the positioning of the laser beam, and focusing errors.
Many diode-pumped solid-state lasers used in laser processing systems are linearly polarized. Certain laser processing systems use circularly polarized laser beams rather than linear polarized laser beams.
One aspect of the invention features a laser polarization control apparatus that includes a polarization modifying device, such as a liquid crystal variable retarder, and a controller. The polarization modifying device receives a laser beam and modifies the polarization of the laser beam. The controller, which is connected to the polarization modifying device, adjusts an input to the polarization modifying device in order to control modification of the polarization of the laser beam based on alignment of a structure to be processed by the laser beam. For example, the polarization of the laser beam may be rotated to correspond with the alignment of a link in a semiconductor device to be cut by the laser beam. The polarization modifying device is configured for incorporation into a laser processing system that produces the laser beam received by the polarization modifying device and that focuses the laser beam modified by the polarization modifying device onto a workpiece that includes the structure to be processed by the laser beam.
Thus, according to the invention, a linearly or elliptically polarized laser beam may be aligned with a link to be cut. For example, the polarization of the laser beam may be vertically aligned when the link is aligned vertically and may be horizontally aligned when the link is aligned horizontally. It has been discovered that by utilizing this technique it is possible to increase the range of acceptable cutting energies that are effective for cutting certain types of links. This range of acceptable cutting energies is the energy window. Thus, by switching the polarization of the laser beam depending on the link orientation, the best results are obtained in terms of maximizing the width of the energy window. There may also be certain types of links for which the energy window is maximized when a linearly polarized laser beam is aligned perpendicularly to the link, or at some other angle, rather than parallel to the link.
Another aspect of the invention features an analyzer tool that receives the laser beam modified by the polarization modification device. The analyzer tool measures the modification of the polarization of the laser beam by the polarization modification device. A plurality of inputs are applied to the polarization modifying device to control modification of the polarization of the laser beam, and the laser beam modified by the polarization modification device is analyzed using the analyzer tool in order to measure modification of the polarization of the laser beam by the polarization modification device. The relationship between the inputs to the polarization control device and the modification of the polarization of the laser beam is stored. When the laser system is used to process a structure, the polarization modification device may modify polarization of the laser beam based on this stored relationship.
By applying a variety of inputs to the polarization modification device and by analyzing the laser beam modified by the polarization modification device, it is possible to identify the appropriate inputs required to obtain vertical linear polarization and horizontal linear polarization, for example. These inputs can then be stored for later use when the laser system processes links on a semiconductor device, so that the appropriate inputs can be applied to the polarization modification device to ensure that the polarization of the laser beam will be vertically aligned when the link is aligned vertically and horizontally aligned when the link is aligned horizontally.
Numerous other features, objects, and advantages of the invention will become apparent from the following detailed description when read in connection with the accompanying drawings.